Clay amount and distribution influence organic carbon content in sand with subsoil clay addition

Sandy soils cover large proportions of Australia's cereal-cropping region and are wide-spread globally. Sandy soils often have low organic C (OC) content due to limited plant growth and rapid decomposition because of low clay concentration. The addition of subsoil clay to sandy soil has the potential to increase OC storage. This study aimed to (i) compare OC stock in a range of clay-amended and unamended sandy soils under cereal cropping and (ii) identify factors that influence OC stock to develop best practices to increase OC storage in clay-amended sandy soils. The study was carried out on four agricultural properties in South Australia and assessed soil OC content, clay content and select physical and chemical properties of clay-amended treatments compared to unamended sands. Clay-amendment treatments differed in the method of clay amendment (clay spread and delved), depth of incorporation (shallow and deep) and amount of subsoil clay added to the surface 30 cm of sand. For each site and treatment, ten soil cores to 50 cm depth were collected within a 25 m grid. There was a positive correlation between OC and clay stock, but it only explained 46% of the variation in OC stock. This indicated that other factors influenced OC stock in clay-amended treatments. Even vertical distribution of clay within the surface 30 cm was a key factor to increase OC storage in clay-amended soils. Subsoil clay properties and the amount added to the surface 30 cm, as well as depth to undisturbed subsoil also influenced OC stock.